In order to increase this you need to make a change to both Bugzilla and MySQL
Change the maxattachmentsize parameters in Bugzilla
In Admin login, go to "Parameters" >> Attachments >> maxattachmentsize, here you can change the Attachment Size as your wish as shown in the screenshot below
Change the size of allowed allowed packets in MySQL
Go to your the max_allowed_packet setting in your mysql my.cnf file:
Saving attachment to filesystem:
The value of parameter "maxattachmentsize" will allow the user to save the attachment into mysql DB and anything above this value and less than the value specified in parameter "maxlocalattachment", bugzill will save the attachment on the filesystem, so adjust the parameter as per your need.
Now if user want to attach the file having size more than the value specified in "maxattachmentsize" parameter, user need to enable the "BigFile" option while adding the attachment, in this case system will save this file on filesystem under bugzilla installation folder "/data/attachment"